Pothole, also known as either a giant's cauldronor or kettle, moulin pothole, or glacial pothole, is a typically large and cylindrical pothole drilled in solid rock underlying a glacier either by water descending down a deep moulin or by gravel rotating in the bed of subglacial meltwater stream.
Wikipedia

Madara Island, one of the islands that belong to Ojika town, has the second-largest pothole in the world that is 3 meters deep and 90 cm in diameter (60 cm around the bottom)! Locals believe that this is a sacred place, building white torii gates to signify this. The white gates contrast beautifully with the blue sea in the background.

NOTE

-Please do not try to approach the pothole in wintertime due to the rough and dangerous sea.